Ultra high frequency powerselective protective device



G. L; TAWNEY 2,540,148

ULTRA HIGH FREQUENCY PowEE-sELECTIvE PROTECTIVE DEVICE Peb. 6, 1951 Filed March 22, 1945 R-T BOX /20 Patented Feb. 6, 1951 UNITED STATES PATENT OFFICE ULTRA HIGH FREQUENCY POWER- SELECTIVE PROTECTIVE DEVICE Delaware Application March 22, 1945, Serial No. 584,092

The present invention relates to ultra high frequency apparatus, and particularly to powerselective resonator electric discharge apparatus. Power-selective apparatus may be employed to advantage in various types of high-frequency circuit applications, among which are included radio object detection systems, sometimes referred to as Radar Systems.

In ultra high frequency radio object detection systems of the type adapted to transmit pulses of ultra high frequency energy toward a remote object and to receive energi7 pulses reflected therefrom, the position of the remote object is determined in accordance with the direction of energy transmission and reception, and in accordance with the time delay between transmission of an energy pulse and the reception of a corresponding reflected energy pulse. In such systems, a single directive antenna usually is employed alternately for transmission and recaption of ultra high frequency energy. The ultra high frequency transmitter may be coupled directly to the antenna so that, during periods of generation of the pulses, energy from the transmitter is supplied to the antenna and is directed toward a remote object. During the intervals between successive transmitter pulses, energy reected from the distant object and intercepted lby the antenna is supplied to the ultra high frequency object detection receiver. Thus, it is readily apparent that the common directive antenna must be coupled to the receiver as well as to the transmitter, and thus, in the absence of a suitable protective device, the sensitive radio object detection receiver would be subject to damage due to the tremendous transmitter output energy during transmission of energy pulses.

In order to prevent excessive power from the transmitter from being conducted to the receiver during the periods of energy pulse generation by the transmitter, a power-selective device such as a T-R box usually is connected intermediate the directive antenna and the ultra high frequency receiver. The usual type of T-R box, or transmitter-receiver ultra high frequency energy blocking device, is a cylindrical cavity resonator having reentrant poles forming a relatively short, high-capacity, high voltage-gradient discharge path. A resonator of the reentrant type which usually forms the basis of the T-R box is illustrated in Fig. 131 (i) of section 3, page 265, in the Radio Engineers Handbook, F. E. Terman, McGraw-Hill, 1943.

The T-R box of the common type usually is 3 Claims. (Cl. 315-39) pressure, particularly adapted to provide an electric discharge path between the reentrant poles of the resonator so that excitation of the resonator produces a short and relatively straight discharge between the reentrant poles of the resonator. Such a T-R box requires relatively high-intensity excitation for the formation of a discharge and therefore it permits the low-intensity energy reflected from a remote object and intercepted by the antenna to be transferred to the receiver. During the transmission of highintensity pulses generated by the transmitter, however, an electric discharge occurs within the T-R box. By virtue of this discharge, the Q of the resonator is'greatly reduced, and thus its eectiveness as an energy-coupling resonator is similarly reduced. Whereas the resonator serves as an energy coupling device for lowintensity energy, it serves as a greatly mismatched impedance, energy-reflecting device for high-intensity energy, and thus it serves to prevent the receiver from being damaged by highintensity energy pulses produced by the transmitter.

Usually, a radio object detection system of the type described above includes a second powerf selective device, commonly referred to as an R-T box, which may be similar in form to the T-R box described above. The R-T box may be coupled to the transmitter and to the antenna at a point intermediate the transmitter and the T-R box connection, for preventing an appreciable part of the reilected energy intercepted by antenna i4 from being diverted from the path through the T-R box to the receiver and being dissipated in the transmitter.

A major disadvantage of power-selective devices such as T-R boxes and R-T boxes of the usual type resides in the fact that the tips of the reentrant poles between which the discharge occurs become heated during the discharges due to transmitter pulses. Such reentrant pole tips may be heated to such a temperature as to cause the release of gases which may mix with the readily ionized gas normally used, so changing the nature of the gaseous medium within the device as well as the pressure thereof as to impair the performance of the power-selective device.

Another disadvantage of the reentrant resonator T-R box or R-T box resides in the fact that the maximum Q of the reentrant resonator is relatively low, so that appreciable attenuation. of a received signal is Vproduced by the powerlled with readily ionized gas at a relatively low 56 selective devices, resulting in a reduction of the range f operation of the object detection system. A further result of the relatively low Q of the reentrant type of resonator is the fact that this type of resonator provides relatively low frequency-selectivity, whereas in some instances it may be desirable to provide appreciable frequency-selectivity `inthe `Ipowerselective devices. A power-selective device capable of high frequency-selectivity aids materially in preventing interference t0 the radio object detection receiver due to transmitters intended `for jamming, or interfering with radio .object-detection for military purposes.

Accordingly, it is an objectnf 'thelpresent invention to provide an improved power-selective device or transmitter-receiver switch for radio object detection systems and other 'ultra :high frequency radio systems. Y

Another object of the present invention 'is to provide a power-selective device of very high frequency-selectivity, and of relatively lowinser- :ation loss or attenuationof received signals.

A further object of the present invention .is to provide a power-selective device having .an ionizable gas medium sealed within a cavity resol:nator adapted to support an annular electric .dis-

charge path.

In a preferred embodiment-.of the present invention, a cavity resonator adapted to support ultra high irequency'oscillaticns in a l.generally -circular-electricneld mode isemployed .as a'pow- :er-selective device. Preferably, a cylindrical resonator is employed, which may be made tunable, ifrdesired. The resonator isfexcited lby the introduction of an ultra high VYfrequency electric iield tangent'ia'lly at vthe cylindrical wall of the resonator, .so that an annular electric -iield is formed within the space inside the resonator.

If desired, a cylindrical resonator unit having xsolid dielectric-lled gas-tight windows for ingress and .egress of ultra high `frequency `energy Icould be filled fwith a suitable readily ionized gas, vacuum pumped, and sealed at a relativ;ly low gas pressure. In a preferred form'of the present invention, .a gas-filled Aglass or other di- --electric .envelope is inserted within a *cylindricall resonator having no reentran't'poles 'and adapted for excitation in .a circular electric mode as by an input waveguide coupld thereto through a .slit or window in the cylindrical resonator wall.

Such a resonator is characterized by a much higher Q than that of vthe usual reentrant-tgype resonator Ahaving a discharge path along its -a-xis. Accordingly, the T-R box of the present inven- .tion not only .aiTords isolation of 'the gaseous electric discharge medium from contact with `the metal walls of the resonator, but lalso provides higher Q v:for 'increased yselectivity and reduced insertion loss.

`An advantage of the above-describedconstruction is the lfreedom from a requirement of ultra high frequency current-carrying connections between the vremovable gas-nlled envelope and the -cylindrical resonator. This feature would be realized even if `there were provided within the path within the resonator remote from any metallic wall or electrode thereof.

The above features will be illustrated and iurther objects will be made apparent by reference to the following detailed description of the present invention, taken in conjunction with the drawing, wherein:

Fig. l illustrates a fixed-tuned 'circular electric mode power-selective device shown connected in a radio object detection system; Fig. 2 is a -modied form of the present invention including a tuning device and a window through the conductive wall of the cavity resonator -whe1'ebylight produced by the electric discharge is emitted ,from the power-selective device; and

Fig.. 3 shows a anodied form of the present invention including a keep-alive electric discharge circuit.

Referring now to Fig. 1, a radio object detection system is shown including an ultra high frequency transmitter I I coupled through transmission lines such'as coaxial lines I2, i 2 and I3 `to a directive antenna I4. The directive antenna I4 may comprise a ydipole antenna element I5 lpositioned at the focal point of a paraboloidal metallic reiiector I6. The transmitter II is adapted to produce ultra high frequency output vin recurrent pulses.' The antenna I4 may be-employed not only for the transmission of energy from transmitter I I, but also for the interception of energy which may be reflected back toward the object detection system from distant objects. For this purpose, the antenna I4 may be cou-pled to an ultra high frequency receiver 2| 'through an energy conducting path comprising a coaxial line 22 'connected to transmission lines i2 and I3 at the junction I9 thereof, a T-R box input wave guide 23, a T-R box 2Q, an output ywave guide 25, and a coaxial line 26. At the end opposite junction I 9, the coaxial transmission line 22 is coupled to the wave guide 23 in a manner well known in the art, and the coaxial line 2E for delivering energy to the receiver 2l is similarly coupled tothe T-R box output wave 'guide 25.

The T-R box 24 is a power-selective device for conducting energy to the receiver 2i during periods of quiescence of transmitter II, but is characterized by an 4electric discharge resulting 'in a great impedance mismatch producing energy reiiection during periods of high intensity output from transmitter I I. The length and characteristics ofthe coupling elements including the wave guide 23 and the coaxial line section 22 are xed so that for low-intensity energy intercepted by antenna I4, an optimum impedance substantially matching the impedance of the antenna Il! and transmission line I3 is presented at the junction I9 by line .22; `while for high-intensity energy lfrom transmitter II, the input impedance presented by line 22 at junction I9 is very much iarger than the impedance of antenna Iii and transmission line I3.

A further power-selective device similar to T-R 4box 24 may be employed as an R-T box 20 coupled through a transmission line section l2" to the junction 3G of transmission lines I2 and I2. The length of the transmission line section I2 and the coupling to the R-T box 2Q is so fixed as to present a very low impedance at junction `313 during periods of low-intensity energizaticn of box 20, but to present a rela- :tively high impedance at junction 30 during an electric discharge in. the R-T box lZIJ produced by high-intensity energy from transmitter Il. In addition, the length of the transmission line section I2' between junction I9 and junction 3U preferably is made equal to one-quarterv of the wave-length vof the energy transmitted and received by the radio object detection system.

During periods of high-intensity energy output from the transmitter I I, the R-T box 20 and the T-R box 24 present very high impedances at junctions 3i) and I9 to the transmission line I2, I 2' and I3 through Which the energy is conducted to the antenna I4. Accordingly, only a small fraction of the transmitter output energy is diverted to the R-T box and the T-R box, this fraction of the transmitter output energy being suicient to sustain the electric discharges in the gaseous media within the boxes. During periods of quiescence of the transmitter I I, the very weak signals intercepted by antenna I4 from detected objects is far below the level required to produce electric discharges within the power-selective devices 20 and 24. The R-T box 2i! therefore presents a very low impedance at junction'l-li), so that, by the well-known impedance transformation Within a quarter-wavelength transmission line, the section I2' extending from junction 30 to junction I 9 presents a very high impedance at the junction I 9 in shunt with the line sections I3 and 22. Accordingly, since the receiver 2I coupled through transmission line section 26, wave guide 25, the T-R box 24, wave guide 23 and transmission line section 22, presents an impedance at junction I9 matching the impedance of antenna I4 and transmission line I3, the greater part or" the energy intercepted by the antenna I4 is sent through the T-R box 24 to the receiver 2|. From this, it is seen that the R-T box 20 prevents an appreciable part of the energy-inter cepted by antenna I4 from being diverted through transmission line I2 and dissipated in the transmitter II. f

Radio object detection systems of the type generally described above are well known and, accordingly, it is not necessary to describe the transmitter and receiver in` detail here. The features of the present invention are embodied Within the improved power-selective devices `2l! and 24. T-R, box 24 is shown in detail to illustrate these features.

T-R box 24, constructed in accordance with the present invention, comprises a cavity resonator having a cylindrical conductive vvall 28 and conductive top and bottom plates 21 and 3|. .Two longitudinally extending slits or Windows 32 and are provided within the cylindrical wall 28 y respectively for coupling energy from Wave guide 23 into the cavity resonator 28, 21, 3I and for permitting the exit of energy from the resonator into the output wave guide 25. The internal dimensions of the cavity resonator 28, 21, 3| are determined relative to the wavelength of the transmitter II for supporting a circular electric field rnc-de of oscillation. For this purpose, the inside diameter of the cylindrical wall 28 may be determined in accordance with Well-known formulae for wave guides operated in any TEM mode, and the length thereof between the top and bottom plates may be made equal to an integral number of half-wavelengths at the wave energy propagation rate in the TEM mode wave guide. Preferably, this length is made equal to one half-wavelength in the interest of compactother solid dielectric which is filled with a readily ionized gas and pumped to a predetermined gas pressure usually lower than atmospheric pressure, for facilitating the formation of an electric discharge along annular path 34. A suitable gas is a mixture of hydrogen and water vapor, the former predominating, and the mixture having a pressure of the order of 25 millimeters of mercury.

In order to maintain a supply of ions in the gas within envelope 35, it is'desirable that a relatively thin layer 36 of radioactive cobalt, or other radioactive material, be deposited on a surface of the glass envelope 35.

At the initiation of an output pulse from the ultra high frequency transmitter II, a high-intensity tangential electric field is produced across the slit 32. by the electromagnetic wave energy transmitted through coaxial lines I2, I2 and 22 and wave guide 23, and a very high-intensity circular electric field is thus produced Within the T-R box 24. This results in a voltage breakdown or electric discharge within the annular path 34 within the resonator, with the result that the Q of the resonator is reduced to an extremely Vlow value. As a result ofthe electric discharge within the T-R box 24, and the sharp diminuation of the Q of the resonator, the resonator becomes greatly mismatched to the wave guide 23, and thus energy arriving through the transmission line section 22 and the wave guide 23 is reiiected at the slit 32 back toward the transmission line 22 and the junction I9, producing the very7 high impedance at junction i9 mentioned above, and the receiver 2I is accordingly protected from damage.

During intervals between high-intensity pulses generated by transmitter I I, reflected energy intercepted by the antenna I4 is transmitted through coaxial lines I3 and 22 and wave guide 23 to the T-R box 2d. and. being of extremely low intensity. is insuicient to produce an electric discharge within. the resonator. Accordingly, the Q of the resonator remains extremely high, so that eiiicient energy transfer is provided from the input wave guide 23 to the output wave guide 25. and the receiver 2I is thus provided with an almost unattenuated version of the signal energy intercepted by the antenna I 4'.

The'radioactive material 36 deposited on the inner surface of the glass envelope 35 serves, as mentioned above, to maintain a condition of ionization within the T-R box 24 at all times, and thus to accelerate the formation of an annular electric discharge within the T-R box 24 upon the commencement of a high-intensity, ultra high frequency energy pulse generated by transmitter I I.

' For some Ipurposes it may be relatively inconvenient to employ a fixed-tuned power-selective device of the .type shown at 24 in Fig. l. As shown in Fig. 2, a variable tuning structure 4I may be incorporated within a T-R box 24. The structure 4I includes a xed top plate 42 having a threaded boss 4'3 thereon, an adjustable tuning screw 44, and an axially movable tuning plate or disc 45 supported by the tuning screw 44. A glass envelope 35 having therein a deposit 36 of radioactive material such as radioactive cobalt may be used just as in the power-selective device shown in Fig. l. By rotating the tuning screw 44 in a direction to raise the plate or disc45, the resonant wavelength of the power-selective device is increased and, accordingly, its resonant frequency is decreased. Conversely, rotation of 7 thescrew M in :the vopposite direction increases the resonant frequency ofthe T-R box 2d.

YIf desired, an'arcuate vslit or windowll'l may be provided in the cylindrical wall 23 of the resonatorZdvso that, during the formation of an electric discharge within the power-selective device, some light Ygenerated by the discharge may escape through the slit il? to serve as an indication of the operation of the device.

An advantage of the circular electric field mode of excitation of 'the power-'selective device is the obviation'of low-resistance, high-frequency current-carrying electrical contact between the tuning disc 45 and the inner surface of the cylindrical Wall 28'. In fact, since conduction lcurrents on `the inner surface of the cylindrical w'all 2B should lcomprise only circular currents, i. e., should be characterized by the absence of any axial components. it may be desirable to provide an annular gap E8 `between the inner surface of the' wall 28' and the outer edge of the disc 45. Such a gap interferes with the response of Vthe cavity resonator to modes ofexcitation other than the circular --Voltage modes, and thus insures best performance of 'the resonator. For the purpose of providing vabsorption of any yenergy which may leak into the space' above' the tuning disc d5, -the upper surface of this disc, as Well 'as vthe Ylower surface 'of the top Vplate d2, may be thickly coated' with ultra high frequency energy dissipat-ve material'su'ch as carbon, for example.

Although radioactive materialv Se" has been shownv in the power-selective devices illustrated inFigs. land 2, such radioactive material may be replaced by 'an-electric discharge circuit, if desired, without iin-pairing the high-Q feature of the present invention. As shown in Fig. 3, a power- 'selective device 24, vgenerally similar to that shown in Fig. 2, is provided with'an electric discharge circuit including keep-alive electrodes 5| and 52 lsupported near the middleV of the space within the 'glass envelope 35 as by `seals 53 and 54 therein. Slit 4'7 serves to transmit light from the device 2li" during the formation of the electric discharge. Suitable sockets55 and 56 for cooperating with extension pins 57 `and 5S fof the gas discharge keep-alive electrodes are provided in the bottom plate-3l" of the power-selective device and are insulated therefrom as by insulating bushings 59 and 6 I. The socket connector elements 55 and 56 maybe connected to a keepalive energization circuit including a source S3 and a series resistor 6d for limiting the discharge current produced within the keep-alive glow discharge path between electrodes 5I and 52. A greater number of keep-alive electrodes may be provided within the gas-filled envelope if desired.

From the foregoing description of the present invention, it is seen that a cavity resonator adapted for circular electric field mode excitation is used as a power-selective device, which may take the form of a T-R box for conducting low-intensity energy to a receiver, and for blocking highintensity energization of the receivers. Another power-selective device of the present invention may be employed as an R-T box. As a result of the use of a circular electric field resonator, it is possible to produce an annular electric discharge within a gaseous medium contained in the resonator, and such a discharge may be made remote from any metal part of the resonator. Along with the elimination of discharge-pathdening electrodes and high-frequency connections thereto, the present invention provides a power-selective device characterized by much higher QLth'anwas'formerly obtained, resulting in Yimproved selectivity and decreased insertion loss.

As shown in .the drawing, the preferred embodimentsfof .the 'power-selective devices constructed in accordance With the present invention aremade with circular cross-section cylindrical side walls and disc ends. Such a structure probably represents the simplest form of the present inventionfor manufacture, and is a very efficient type of construction for providing the features for the present invention. However, the generally circular 4electric field resonator may take other forms, such as a cylinder having an elliptical cross-section. The end plates may be nat or may be curved upwardly or downwardly, or a hollow sphere could be employed for the resonator. The present vinvention is distinguished from former devices principally in the factthat the highintensity electric field within the resonator is a continuous field within the gas therein so that it is not required to extend from one wall to another wall of the resonator. Accordingly, as pointed out above, contact difficulties are eliminated, and the Q' and the ehiciency of the powerselective device are greatly improved.

Thus, where in vthe appended claims the term .cylinder is employed, ythis term is to be construed in its broadest sense as being a surface generated by the extension along an axis of a 'closed gure, Iso that the. cross-section of the cylinder, as referred to hereinafter, may be circular, elliptical or otherwise, as desired. Similarly, where in the claims the end walls of the resonator are referred to as disos, it will be understood that these' terms are not intended to be restricted to elements characterized by planar surfaces, but may be curved upwardly or downwardly, without departing from the spirit of the present invention. Where a circular electric field mode of oscillation is referred to, it will be understood that this term is meant to define a eld characterized by ring-like lines of electric force, the high-intensity lines of force being isolated from the wfalls of the cavity resonator Within which the oscillations are produced. Also, by annular electric discharge," the 'claims are intended to refer to an electric discharge which is not necessarily a discharge extending throughout a circular path, but is a discharge along the path of the ring-like lines of electric force referred to above.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A device of the character described comprising a conducting body having a cavity resonator characterized by supporting an electromagnetic wave energy mode having an annular electric field component, said resonator having input and output windows coupling electromagnetic wave energy therethrough; and a device for substantially reflecting high-intensity electromagnetic wave energy elds at said input window to prevent high-intensity oscillations in said cavity resonator, said device consisting in its entirety of a sealed vessel in said cavity resonator containing a body of ionizable gas, and means for maintaining ionization of said gas, said gaseous body supporting an annular electric discharge due to said annular electric eld component when said resonator is energized by said high-intensity waves.

2. A transmitter-receiver switch comprising a conducting body having a cavity resonatorcharacterized by supporting an electromagnetic wave energy mode having an annular electric field component, said resonator having input and out,- put windows for coupling electromagnetic Wave energy therethrough; and means f for substantially reecting high-intensity electromagnetic Wave energy elds at said input window to prevent high-intensity oscillations in said cavity resonator, said means consisting in its entirety of an ionizable gaseous medium, means sealing said gas in said resonator against loss through said Windows, and a radio-active material maintaining ionization of said gaseous medium, said gaseous medium supporting an annular electric discharge owing to an increase in the magnitude of said annular electric field component when said resonator is energized by said high-intensityy waves.

3. Atransrnitter-receiver switch comprising a conducting body having a cavity resonator characterized by supporting an electromagnetic wave energy mode having an annular electric field component, said resonator having input and output windows coupling electromagnetic wave energy lltherethrough; and a device for substantially4 reflecting high-intensity electromagnetic waveenergy fields at said input window to prevent high-intensity oscillations in said cavity resonator, said device consisting in its entirety of a sealed vessel in said cavity resonator containing 10 a body of ionizable gas, and a radio-active material maintaining ionization of said gas, said gaseous body supporting an annular electric discharge in the presence of said annular electric eld component when said resonator is energized by said high-intensity waves.

GERELD LEON TAWNEY.

REFERENCES CITED The following references are of record in the file of this patent:

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